This invention relates to plasma-generating devices. More specifically, this invention relates to magnetron-type plasma-generating devices (that is, magnetrons) capable of sustaining plasmas of more uniform density than plasmas generated by previously known magnetron-type plasma-generating devices.
Magnetrons have been known in the art for a long time and have been used, for example, in etching, surface modification, and plasma-enhanced chemical vapor deposition ("PECVD"). PECVD devices are also known in the art. Examples of PECVD devices can be found in U.S. Pat. Nos. 5,298,587; 5,320,875; 5,433,786; and 5,494,712, (collectively "Hu et al.") the teachings of which are herein incorporated by reference.
As explained in Chapter 6 of the Handbook of Plasma Processing Technology, Noyes Publications 1990, magnetrons are a class of cold cathode discharge devices generally used in a diode mode. A plasma is initiated between the cathode and the anode at pressures in the mTorr range by the application of a high voltage, which can be either dc or rf. The plasma is sustained by the ionization caused by secondary electrons emitted from the cathode due to ion bombardment which are accelerated into the plasma across the cathode sheath. What differentiates a magnetron cathode from a conventional diode cathode is the presence of a magnetic field. The magnetic field in the magnetron is oriented such that a component of the magnetic field is parallel to the cathode surface. The local polarity of the magnetic field is oriented such that the ExB drift paths of the emitted secondary electrons form a closed loop. Due to the increased confinement of the secondary electrons in this ExB drift loop compared to a dc or rf diode device, the plasma density is much higher, often by an order of magnitude or more, than a conventional rf or dc diode plasma. The result of the high plasma density and its proximity to the cathode is a high current, relatively low voltage discharge.
Hu et al. teaches a method of forming a protective abrasion resistant coating onto a substrate surface. In the method taught in Hu et al., a PECVD method utilizing a magnetic confined electrode is used to initiate the polymerization reaction of an organosilicone compound and excess oxygen employing a power density ranging from 10.sup.6 to 10.sup.8 Joules (J)/Kilogram (Kg), in the presence of a substrate having a suitable surface to cause the polymerization product of the plasma process to adhere to the substrate surface. In Hu et al., the magnetic confined electrode utilizes magnets having sufficient strength to provide at least 100 gauss.
It is also known in the art, that when using a magnetron in a process to coat a substrate such as in a PECVD process, it is difficult to obtain a coating of uniform thickness and quality. One aspect of quality is uniform chemical composition of the coating both in thickness and width directions. To get a coating of uniform thickness and quality the substrate must be moved relative to the electrodes. This is especially true for large substrates. Moving the substrates relative to the electrodes causes a decrease in throughput.
The present invention allows for more uniform (thickness and quality) coatings to be obtained more easily than do devices of the prior art, especially on large substrates. In one aspect, the present invention is an electrode containing multiple magnets positioned such that like magnetic poles of said magnets are all facing in substantially the same direction. Each magnet produces a magnetic field between the opposite magnetic poles on the same magnet. Each magnetic field has a component parallel to the surface of the electrode. Electrodes of the present invention have a higher number of closed loop ExB drift paths per number of magnets than electrodes of the prior art. Electrodes of the present invention are capable of producing a more uniform plasma across the surface of an electrode. In addition, electrodes of the present invention produce plasmas of greater height than electrodes of the prior art. According to the present invention, large numbers of magnets (that is, two or more) can be aligned in various configurations so as to create large electrodes capable of producing large, more uniform, plasmas.
In another aspect, the present invention is an improved plasma-generating device utilizing electrodes of the present invention. In yet another aspect, the present invention is an improved method of forming a plasma and an improved method for coating various substrates.
In one embodiment of the present invention, the electrode is a planar electrode comprising two or more magnets positioned such that like poles of said magnets are in a single geometric plane parallel to the geometric plane of the planar electrode and the polarity of said magnets is perpendicular to the geometric plane of the planar electrode, each magnet producing a magnetic field having a component parallel to the geometric plane of the electrode.